First generation bioethanol is mainly produced on the basis of cereal crops like wheat and maize as well as sugar cane. This is due to the fact that corn and sugar cane contain readily accessible carbohydrates such as starch that can be converted into sugar in a simple way, and which is subsequently is fermented into ethanol.
However, this production has been critisised for converting good foodstuffs into energy apart from not being sustainable. For some years, research has therefore been made into utilising crop residue from food production for production of biofuel, in particular bioethanol. Research has particularly concentrated on converting straw and wood chips into bioethanol. This type of ethanol is labelled second generation bioethanol or cellulosic ethanol.
Biomass, such as wheat straw and straw from other corn and maize crops and wood, consists largely of cellulose, hemicellulose and lignin why it is also collectively called lignocellulose.
Cellulose is a linear homogenous polymer of up to 15,000 glucose units interconnected by β-1,4-glucoside bonds. Hemicellulose is, however, a heterogeneous branched polymer with a length up to 200 units which can consist of e.g. arabinose, xylose, galactose, mannose and glucose.
Lignin constitutes a network formed by polymerisation of the monomers p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol. The complex network of lignin encapsulates and contributes to binding cellulose and hemicellulose together. The structure of the plant cell wall is hereby strengthened and protected against decomposition in the nature e.g. by attacks from fungi or insects. In general, lignocellulose contains about 35-50% cellulose, 20-30% hemicellulose and 15-30% lignin.
However, there are great differences in the contents of various plants and the composition of hemicellulose and lignin is very dependent on the species. In general, wood contains more lignin and less hemicellulose than straw, and where hemicellulose in straw mainly consists of arabinose and xylose, in conifers it contains mostly mannose and only a little xylose.
Utilisation of lignocellulose as substrate for various fermentation processes presupposes a prior decomposition of cellulose and hemicellulose into their respective monomers. The first step in this process is a thermochemical treatment of the lignocellulose whereby lignin is released and hemicellulose and cellulose are partially dissolved or made more accessible to enzymes.
The enzymes for decomposing lignocellulose can be divided into two main groups: cellulases and hemicellulases. The last step in the decomposition of cellulose is the cleavage of cellobiosis into two glucose molecules by the enzyme β-glucosidase. The more heterogeneous structure of hemicelluloses means that a greater number of different enzymes are required to completely decompose it into sugar molecules. An example of such a complex enzyme mixture is Novozyme's Cellic. CTec3 which contains different cellulases and hemicellulases as well as other hydrolytic enzymes.
As mentioned, the first step in the utilisation of lignocellulose is a pre-treatment and typically a thermochemical pre-treatment. Steam explosion is one of a wide range of such different thermochemical pre-treatment methods. This process is combined with addition of water and catalysts such as acids and bases or gases like oxygen and sulphur dioxide.
Pre-treatment of biomasses like straw and wood chips for making fluid bio-fuels, especially ethanol, has been subject to a very comprehensive research effort, and a massive amount of scientific literature is thus available on this area.
In the recent years dominating biochemical methods have been described. A comprehensive presentation of these works is not to be made here, but it is, however, to be noted that several groups point to autohydrolysis as the preferred technology because it is not based on chemicals, because formation of inhibitors is modest and because biomass with relatively high dry matter content can be processed. It is also preferred by most authors over wet oxidation in which oxygen is added to the process.
Autohydrolysis is termed differently but is often called thermal hydrolysis, steaming or steam explosion regardless that the explosion part is not necessarily an advantage to the hydrolysis or comminution of the material. The method borders to “liquid hot water treatment”, depending on the amount of water, and wet oxidation if oxygen forms part of the process.
The scientific literature furthermore points to the use of a number of chemicals and catalysts or to hydrolysis of lignocellulose, including weak and strong acids and bases and a number of gases like SO2, CO2, O2, NH3, H2O2, O3. To this is added application of enzymes, either made industrially or as a biological pre-treatment.
The technical installations used for such thermochemical pretreatments of lignocellulose-containing biomasses have only been made in a few examples.
The best known apparatus is the staketech hydrolysator of SunOpta which is used in the first commercial plant for producing bioethanol based on straw. This machine has a horizontal reaction chamber with a screw conveyor moving the straw forward under high pressure and temperature, allowing it to explode into an associated expansion container at frequent intervals, i.e. at intervals of a few seconds. The operating temperature and pressure are 190-210° C. and 15-20 bars, respectively.
The Atlas Stord hydrolysator for hydrolysis of feathers uses a different principle, so-called plug flow, where the reaction chamber is a vertical chamber with a valve at the bottom which opens and closes at intervals of a few seconds. The overpressure in the reaction chamber will thus make the hydrolysed feathers to explode into an expansion container. The reaction chamber is therefore not provided with a shaft passage. The operating temperature and pressure are 160-210° C. and 6-10 bars, respectively.
Finally, Villavicencio (1987) has published an invention for thermochemical treatment of fibres by means of several reaction chambers. The biomass is supplied via screw conveyors, which also act as back pressure valves, to the first reaction chamber.
Common to all techniques are that                1) heat is supplied from an external heat source, particularly by means of hot water or steam;        2) water in the form of liquid water or steam is added to the process such that the dry matter content is at most 30-40% in the reaction chamber, and typically 10%;        3) water or steam is added as a necessary prerequisite for treatment at high temperatures at the level of 160-220° C.        
The operational mode of the technique, and as the name “steam explosion” indicates, is a mechanical decomposition of the fibres of the biomass by a steam explosion caused by a sudden pressure drop from e.g. 20 bars to atmospheric pressure. The state of water at e.g. 200° C. under pressure is as a liquid, but when the pressure abruptly drops to atmospheric pressure, part of the water is transformed into steam, meaning the water occurring in all parts of the plant fibres as well. When this water explodes in the cellulose fibres, the biomass is torn up mechanically. This tearing up contributes to make the component parts of lignocellulose of cellulose and hemicellulose accessible for further processing, as for example by enzymatic decomposition.
Conventional steam explosion is often accomplished at temperatures in the range 160-220° C. and corresponding pressures at 0.60-4.83 MPa. The processing time varies from a few seconds to several minutes before the material is exposed to atmospheric pressure via explosive decompression. The process causes decomposition of hemicellulose and transformation of lignin due to the high temperature. Hemicellulose is decomposed by acetic acid and other organic acids formed during the treatment, i.e. via so-called autohydrolysis. Lignin is not decomposed to the same degree but is redistributed on the fibre surfaces as a result of melting and depolymerisation/repolymerisation reactions.
Besides these chemical effects, steam explosion also has a purely mechanical or physical effect as the material explodes and fragments whereby the accessible surface is increased.
The procedure is implemented, as mentioned, by adding water to the biomass, either in the form of liquid water or in the form of steam, or a combination thereof, and heating the mixture. High temperatures are attained by heating with hot water or steam.
The highest dry matter concentration achieved by these systems is about 30-40%, typically much lower, requiring large technical installations due to the amount of water and the voluminous structure of the biomass. Even a compressed straw bale has a density of about 150 kg/m3 which is not much.
A crucial challenge to the technique is the large amounts of water and energy used for pre-treatment and the necessarily large installations for pressure containers, valves, pipes, screw conveyors etc.
This also entails substantial drawbacks by biogas plants since the large addition of water with the straw strains the hydraulic capacity of a biogas plant, and since the energy consumption reduces the net energy production and the cost efficiency.
A biofuel can also be provided in the form of biogas. Until now biomass, preferably in the form of straw, has not been used for biogas production. It is not known to use straw for biogas purposes. It is only known that straw forms part of biogas plants to the extent that straw is used as bedding in livestock production and to the extent that the resulting livestock manure is degassed.
Actually, it is rather surprising that straw is not used for biogas purposes. In the light of the fact that livestock manure, i.e. essentially cattle and pig liquid manure, is fluid with a dry matter content between 4 and 8%, there is room for additional dry matter in the biogas plant, in particular straw.
Straw is a difficult material to handle. It is very abrasive, very hydrophobic and has a very low density, i.e. less than 100 kg per m3. The handling of straw in any connection and in particular in biogas plants therefore requires a special technique.
In addition, straw predominantly consists of cellulosic fibres which are crystalline polymers of (1-4)-β-D-glucose. Hemicellulose forms part thereof which correspondingly is an amorphous and partly crystalline polymer consisting of (1-4)-β-xylose. Hemicellulose forms part of both fibres and cell walls. Lignin, a third essential component of straw, is a polymer of phenol. Hemicellulose as well as lignin protect the cellulose against “weather and wind”, and in this connection against decomposition by enzymes and microorganisms.
In order to efficiently utilise straw in a biogas plant it is thus necessary to pretreat the straw in order to open up the fibres of the straw and to make the component parts of the lignocellulose accessible to decomposition. As mentioned above, this will be energy-consuming and necessitate use of voluminous plants.